FIELD OF THE INVENTION
[0001] The invention relates to an improved printhead design for an ink jet printer and
to a method for reducing thermal and/or mechanical stress in a composite printhead
structure.
BACKGROUND OF THE INVENTION
[0002] Ink jet printheads are composite structures which are conventionally made by bonding
a metal or plastic nozzle plate to a semiconductor substrate either directly using
an adhesive or by bonding the nozzle plate to a polymeric layer which is deposited
on or bonded to the substrate. The polymeric layer may be patterned before or after
bonding to the substrate in order to provide ink flow features which provide ink to
the regions of the printhead which induce the ink to be expelled through the nozzle
plate to a print media.
[0003] In order to bond a nozzle plate to the polymeric layer, heat and pressure are applied
to the nozzle plate and substrate. Because each of the nozzle plate, polymeric layer
and substrate material often have a different modulus of elasticity and coefficient
of thermal expansion, the materials of the printhead composite tend to expand and
contract at different rates and by different amounts when heated and/or cooled. The
uneven expansion and/or contraction of the components during the bonding process induce
stresses which warp the components thereby causing misalignment and stresses which
increases the tendency for the components to fracture during assembly and use of the
printhead. Component misalignment and/or warpage may result misfiring of the printhead
or in ink being misdirected from the printhead.
[0004] As the number of nozzle holes increases and the size of the holes decreases, the
criticality of component alignment becomes substantially more important for the proper
functioning of the printer. Printhead structures which are warped or which contain
components which are not aligned properly result in significantly reduced printer
performance and quality.
[0005] An object of the invention is to improve component alignment in a printhead structure.
[0006] Another object of the invention is to reduce thermal stresses in print head components
during assembly thereof.
[0007] A further object of the invention is to provide a less costly manufacturing process
for printhead components which induces relatively less thermal stresses in the components
parts thereof.
SUMMARY OF THE INVENTION
[0008] With regard to the above and other advantages, the invention provides a printhead
composite structure including a semiconductor substrate containing energy imparting
devices for ink and electrical tracing connected thereto on a surface of the substrate,
a thick film polymeric layer adjacent the energy imparting surface of the substrate
and a nozzle plate attached to the polymeric layer. The polymeric layer has a sufficient
thickness and size suitable for containing a plurality of ink chambers and ink flow
channels and a plurality of valleys in an area of the polymeric layer adjacent the
ink chambers which valleys are sufficient to inhibit thermally induced stresses in
the polymeric layer during a process for bonding the nozzle plate to the polymeric
layer.
[0009] In another embodiment, the invention provides a method for making an ink jet printhead
which comprises providing a semiconductor substrate containing electrical tracing
connected to energy imparting devices for ink on a surface of the substrate, applying
a polymeric layer onto the surface of the semiconductor substrate, the polymeric layer
having a thickness ranging from about 2 to about 50 microns, preferably from about
10 to about 30 microns, treating the polymeric layer in one or more steps to provide
ink chambers and ink flow channels therein for flow of ink to the energy imparting
devices and to produce valleys adjacent the ink chambers, and bonding a metal coated
nozzle plate adjacent to the polymeric layer using heat thereby forming an ink jet
printhead, wherein the valleys are of a size and located in an area of the polymeric
layer sufficient to minimize thermal stresses in the polymeric layer during the bonding
process.
[0010] In yet another embodiment, the invention provides thermal ink jet printer cartridge
which comprises an ink reservoir body, electrical contacts for connecting the cartridge
to a printer and a printhead structure attached to an electrical tab circuit containing
the contacts, wherein the printhead structure comprises a semiconductor substrate
having thermal resistance elements and electrical traces on a ink wettable surface
thereof and an ink via therethrough, a photoresist thick film polymeric layer attached
adjacent the ink wettable surface of the substrate and a metal, metal coated or plastic
nozzle plate attached to the polymeric layer wherein the polymeric layer contains
a multiplicity of ink flow channels leading from an inlet ink region to ink chambers
adjacent the inlet ink region. The polymeric layer also contains a plurality of voids
in an area of the polymeric layer adjacent the ink chambers, the voids having a size
sufficient to inhibit thermal stresses in the printhead structure during a manufacturing
process therefor.
[0011] An advantage of the invention is that the valleys or voids, which provide reduced
thermal stresses during the process of bonding the nozzle plate to the polymeric layer,
are formed in the polymeric layer rather than in the nozzle plate thereby simplifying
the manufacturing process. Furthermore, the valleys or voids may be produced at the
same time or substantially the same time as the production of other flow features
in the thick film or polymeric layer thereby reducing the number of process steps
as compared to producing a metal or metal coated nozzle plate and forming the valleys
or voids in the nozzle plate using a separate machining step.
Brief Description Of The Drawings
[0012] Further advantages of the invention will become apparent by reference to the detailed
description of preferred embodiments when considered in conjunction with the following
drawings, which are not to scale so as to better show the detail, in which like reference
numerals denote like elements throughout the several views, and wherein:
Fig. 1 is a cross-sectional view from one end of a printhead structure according to
the invention through ink flow regions of the structure;
Figs. 2 and 2A are top plan views, not to scale of printhead structures according
to the invention;
Figs. 3 is a partial side view, not to scale, of a printhead composite structure according
to the invention;
Figs. 4 and 5 are enlarged views, not to scale, of portions of printhead composite
structures according to the invention; and
Fig. 6 is an enlarged view, not to scale, of a portion of thick film polymeric material
illustrating the effect of aspect ratio on the depth of the valley formed in the polymer
layer.
DETAILED DESCRIPTION OF THE INVENTION
[0013] Referring now to the figures, Fig. 1 is a cross-sectional view from one end of a
printhead composite structure 10 according to the invention. The printhead structure
10 includes a semiconductor substrate 12, preferably a single crystal silicon substrate,
which may contain an ink flow passage or via 14 for flow of ink from an ink reservoir
to the energy imparting region of the printhead, generally designated as 16. The invention
is not limited to flow of ink from a central via in the substrate, as the ink may
also be caused to flow around the edges of the substrate into the energy imparting
region of the printhead. The energy imparting region 16 preferably contains resistance
heaters 18A and 18B or other energy imparting devices for inducing ink which has accumulated
in ink chambers 20A and 20B to be expelled through nozzle holes 24A and 24B in a nozzle
plate 26.
[0014] The semiconductor substrate 12 is preferably a single crystal silicon substrate which
is defined as one of a plurality of individual substrates on a silicon wafer. As described
the silicon wafer may be patterned to provide ink vias 14 in each of the substrates
for flow of ink from a reservoir to an ink wettable surface of the substrate. Electrical
tracing and contacts are also deposited on the individual substrates to provide electrical
connection between the energy imparting devices such as resistance heaters 18A and
18B and a printer controller. In order to provide suitable ink flow features, a polymeric
layer 22 is preferably deposited or attached to the wafer so that flow features for
the individual printhead structures can be patterned therein.
[0015] The flow features provided in the polymeric layer 22 include ink chambers 20A and
20B and associated flow channels which are formed may be formed in a central region
of the polymeric layer 22 so that ink flow channels are in flow communication with
the ink chambers 20A and 20B and a central ink inlet region 28 which is in flow communication
from central ink via 14 in the substrate. In the case of ink flow around the edges
of the substrate, the ink flow channels are positioned near the edges of the polymeric
layer 22 and the central ink inlet region 28 is not required. For simplicity, the
printhead structures will be described with reference to a single printhead structure
on the wafer. However, it will be understood that multiple printhead structures are
preferably formed at one time on the silicon wafer and once the structures are complete,
they are removed from the wafer and attached along with the polymeric layer to a printhead
region of a printer cartridge.
[0016] The polymeric layer 22 may be a single or multiple polymeric layer, each layer being
a photoimageable polymeric materials selected from positive and negative photoresist
materials such as polydimethylglutarimide (PMGI)-based photoresists, polymethylmethacrylate
(PMMA)-based photoresists, PMGI-PMMA copolymer photoresists, phenol-formaldehyde-type
photoresists and photodecomposable polymeric compounds derived from vinylketone, or
a laser ablatable material such as polyimide. The polymeric layer 22 may be adhesively
bonded to the substrate 12 as a dry film or may be coated onto the substrate 12 from
a solution using spin-coating techniques. A B-stageable adhesive may be used as the
polymeric layer or as one of the polymeric layers to adhesively bond the polymeric
layer and nozzle plate to one another.
[0017] It is preferred to pattern the polymeric layer 22 with the flow features alter the
layer is applied to the substrate 12, however, the invention is not limited to patterning
the layer 22 alter it is applied to the substrate, nor is the invention limited to
a single polymeric layer. Multiple polymeric layers 22 comprised of the same or different
materials may be used to provide the flow features and other aspects of the invention.
[0018] In order to pattern a polymeric layer 22 made of a photoresist material, the layer
is preferably exposed to a light or electron beam radiation source, preferably an
ultraviolet light source through a mask in a pattern which defines the ink chambers
20A and 2B, the ink inlet region 28 and the ink flow channels. After exposing the
polymeric layer 22 to light sufficient to cure defined areas of the layer 22, the
uncured portions of the layer are removed by dissolving the uncured portions in a
suitable solvent such as a butylcellosolve acetate/xylene mixture. When a polyimide
material is used as the polymeric layer 22, the polyimide is preferably ablated through
a mask using a laser beam source sufficient to remove portions of the polyimide material
thereby defining the flow features of the layer 22. The flow features may also be
patterned on a dry film polymeric layer 22 before the layer is aligned with and fixedly
attached to the substrate 12.
[0019] Once the polymeric layer 22 is pattered, a nozzle plate 26 is bonded to the polymeric
layer 22. The nozzle plate 26 is preferably provided by a gold or a gold-plated nickel
material which contains a plurality of nozzle holes therein. The nozzle holes align
with the flow features patterned into the polymeric layer 22 in order to provide conduits
to direct ink from the ink chambers 20A and 20B to a print media. The nozzle holes
typically have an entrance diameter of about 43 microns on the polymeric layer side
of the nozzle plate to an exit diameter of about 29 on the print media side of the
nozzle plate. A typical nozzle plate may contain from about 50 to about 100 nozzle
holes or more. Considering that the nozzle plate 26 has a length of from about 6 to
about 25 millimeters and a width of from about 2 to about 40 millimeters, preferably
from about 3 to about 20 millimeters, it will be appreciated that even slight misalignment
or warpage of the nozzle plate may have a significant impact on print quality.
[0020] During the manufacturing process, heat and pressure are applied to the nozzle plate
26 and to the polymeric layer 22 on the substrate 12 to bond the nozzle plate 26 to
the polymeric layer 22. Because the substrate 12, polymeric layer 22 and nozzle plate
26 are made of different materials, they each have a unique set of thermal and mechanical
properties. Most notably, the differences in modulus of elasticity and the coefficient
of thermal expansion of each of the materials cause stresses in the materials due
to unequal expansion and contraction of the individual components as the components
are heated and cooled. Because the polymeric layer 22 is attached to the substrate
12 and the nozzle plate 26 is fixedly bonded or adhered to the polymeric layer, the
stresses induced in the components by heat and pressure used for bonding the nozzle
plate 26 to the layer 22, unless compensated for, may cause unwanted warpage or misalignment
of the components. Figs. 2-6 provide illustrations of the preferred methods, according
to the invention, for relieving thermal stresses in the components during a manufacturing
process therefor.
[0021] Fig. 2 is a top plan view, not to scale, of a printhead structure 10 prior to attaching
a nozzle plate 26 thereto which contains a semiconductor substrate 12 and a polymeric
film or polymeric layer 22 attached to a surface of the substrate 12 and which illustrates
the improvements according to the invention. The polymeric layer 22 is selectively
thick, in that it preferably has a thickness ranging from about 2 to about 50 microns,
preferably from about 10 to about 30 microns.
[0022] As shown in Fig. 2, the polymeric layer or film 22 contains a substantially central
region 30 in which flow features for ink as described with reference to Fig. 1 are
contained and an outer region 32 surrounding the central region containing sufficient
valleys, voids or other discontinuities which serve as expansion areas for reducing
thermal stresses produced during the manufacturing process.
[0023] It is noted that in the embodiment of Fig. 2, the outer region 32 completely surrounds
the central region 30 of the polymeric layer 22. However, for the purposes of this
invention, at least side regions 32A and 32B adjacent the central region 30 contain
valleys for reducing thermal stresses while end regions 32C and 32D need not contain
such valleys. Side regions 32A and 32B are between the ink chambers 20A and 20B (Fig.
1) and the edges 34A and 34B of layer 22.
[0024] Fig. 2A illustrates an alternative embodiment of the invention wherein the ink flows
to the flow features of the polymeric layer 22' from the around the edges of the semiconductor
substrate 12'. In this embodiment, the flow features are patterned in the polymeric
layer 22' generally in outer region 30' which extends from the edges 34A' and 34B'
of the polymeric layer 22' to a central region 32' which contains the valleys for
reducing thermal stresses during the process of bonding a nozzle plate to the polymeric
layer 22'.
[0025] A partial cross-sectional view of a side portion of a printhead composite structure
10 according to the invention along view A-A of Fig. 2 is illustrated in Fig. 3. The
printhead composite structure 10 preferably includes a semiconductor substrate 12,
a polymeric layer 22 attached to the substrate 12 and a nozzle plate 26 attached to
the polymeric layer 22. The polymeric layer 22 preferably contains a plurality of
valleys or voids 36 which inhibit thermal stresses in the structure 10 when the nozzle
plate 26 is fixedly attached to the polymeric layer 22 of the structure.
[0026] The valleys or voids 36 may be provided with a variety of shapes such as straight,
curved or sloped walls, and may have a depth at least as thick as the polymeric layer
22 as shown in Fig. 5 (36A) or a depth that is at least 33% of the thickness of the
polymeric layer 22 as shown in Fig. 4. The valleys 36 are preferably formed so that
they lie substantially perpendicular to the longest dimension of side regions 32A
and 32B and substantially perpendicular to the longest dimension of end regions 32C
and 32D (Fig. 2).
[0027] As with the flow features, the valleys 36 may be patterned in the polymeric layer
22 either before or alter applying the polymeric layer to the substrate 12. The same
patterning techniques using a mask may be used to form the valleys 36 as is used to
define the flow features in layer 22. In the alternative, the valleys may be mechanically
abraded in the polymeric layer 22 using a grinding wheel or other abrasive device.
Because the valleys are contained in the polymeric layer 22, there is no need to provide
gaps or surface roughness on the metal or metal coated nozzle plate. Accordingly,
the manufacturing steps for the printhead structure are greatly simplified particularly
since the valleys can be formed at the same time or substantially the same time as
the other flow features in the polymeric layer 22.
[0028] As described above, the valleys 36 need not extend completely through the polymeric
layer 22 to be effective. Accordingly, the depth of the valleys 36 may be controlled
by selecting various aspect ratios for the valleys. The aspect ratio of a valley is
defined as the greatest width of the valley divided by the thickness of the polymeric
material used for the polymeric layer. For example, for a photoresist acrylate material
such as LEARONAL having a thickness of about 30 microns, an aspect ratio of greater
than about 18/30 will provide a valley having a depth equal to the thickness of the
polymeric material. Accordingly, masks having widths of greater than 18 up to about
30 microns will produce valleys which extend completely through the polymeric material.
[0029] The relationship of aspect ratio to polymer layer thickness is illustrated by reference
to Fig. 6. As shown, the polymeric layer 50 has a thickness T of 30 microns. For a
width W of valley 52 of greater than about 18 microns, the depth D of the valley 52
is equal to the thickness T of the polymeric layer 50. However for valley 54 having
a width W' of less than 18 microns, the depth D' of the valley 54 is less than the
thickness T of the polymeric layer.
[0030] While the aspect ratio for the foregoing material requires an aspect ratio of less
than about 18/30 in order to create a valley which does not extend all the way through
the polymeric layer, the particular light source, hardware capabilities, polymeric
materials and other factors may affect the aspect ratio for a particular polymeric
material. Accordingly, one skilled in the art may readily determine the aspect ratio
for any particular polymeric material in order to produce valleys of the desired depth.
[0031] While specific embodiments of the invention have been described with particularity
above, it will be appreciated that various modifications, substitutions and additions
may be made to the invention by those skilled or ordinary skill in the art without
departing from the spirit and scope of the appended claims.
1. A method for making an ink jet printhead comprising providing a semiconductor substrate
containing electrical tracing connected to energy imparting devices for ink on a surface
of the substrate;
applying a polymeric layer onto the surface of the semiconductor substrate, the polymeric
layer having a thickness ranging from about 2 to about 50 microns;
treating the polymeric layer in one or more steps to provide ink chambers and ink
flow channels therein for flow of ink to the energy imparting devices and to produce
valleys adjacent the ink chambers; and
bonding a metal coated nozzle plate adjacent to the polymeric layer using heat to
produce an ink jet printhead,
wherein the valleys are of a size and located in an area of the polymeric layer
sufficient to minimize thermal stresses in the polymeric layer during the bonding
process.
2. The method of Claim 1 wherein the polymeric layer is applied to the semiconductor
substrate by spin coating the substrate with a polymeric material.
3. The method of Claim 1 or Claim 2 wherein the polymeric layer is treated to provide
the ink chambers and ink flow channels by photoimaging, chemically etching or laser
ablating the polymeric layer.
4. The method of Claim 1 or Claim 2 wherein the polymeric layer is treated to provide
the valleys by photoimaging, chemically etching or laser ablating the polymeric layer.
5. The method of any one of Claims 1 to 4 wherein the nozzle plate is bonded to the treated
polymeric layer using heat and pressure.
6. The method of Claim 1 wherein the polymeric layer is a polyimide and the valleys are
produced by laser ablating the polyimide to a depth of at least about 33% of the thickness
of the polymeric layer.
7. A printhead composite structure comprising a semiconductor substrate containing energy
imparting devices for ink and electrical tracing connected thereto on a surface of
the substrate, a thick film polymeric layer adjacent the energy imparting surface
of the substrate and a nozzle plate attached to the polymeric layer, wherein the polymeric
layer has a sufficient thickness and size suitable for containing a plurality of ink
chambers and ink flow channels and a plurality of valleys in an area of the polymeric
layer adjacent the ink chambers which valleys are of a size and located in an area
of the polymeric layer sufficient to inhibit thermally induced stresses in the polymeric
layer during a process for bonding the nozzle plate to the polymeric layer.
8. A thermal ink jet printer cartridge having an ink reservoir body, electrical contacts
for connecting the cartridge to a printer and a printhead structure adjacent to an
electrical tab circuit containing the contacts, the printhead structure comprising
a semiconductor substrate having thermal resistance elements and electrical traces
on an ink wettable surface thereof and an ink via therethrough, a photoresist polymeric
layer adjacent the ink wettable surface of the substrate and a metal or metal coated
nozzle plate adjacent to the polymeric layer wherein the polymeric layer contains
a multiplicity of ink flow channels leading from an inlet ink region to ink chambers
adjacent the inlet ink region, and wherein the polymeric layer also contains a plurality
of voids in an area of the polymeric layer adjacent the ink chambers, the voids having
a size sufficient inhibit thermal stresses in the printhead structure during a manufacturing
process therefor.
9. A printhead for a thermal ink jet printer comprising a semiconductor substrate containing
an ink flow passage for flowing ink from an ink reservoir to an energy imparting region
on the substrate, a layer of polymeric material adjacent the energy imparting region
of the substrate having ink chambers, ink flow channels and an ink supply region therein
cooperating with the ink flow passage to provide ink to adjacent the energy imparting
region of the substrate and a metal or metal coated nozzle plate containing nozzle
holes for expelling ink from the ink chamber to a print media, the nozzle plate being
bonded to portions of the polymeric layer using heat and pressure, wherein the polymeric
layer also contains a plurality of void spaces therein to provide discontinuities
in the polymeric layer which inhibit the formation of thermal stresses in the printhead
structure during bonding process.
10. The printhead of Claim 9 wherein the void spaces in the polymeric layer are between
the ink chambers and at least two opposing edges of the polymeric layer wherein the
void spaces have a depth substantially equal to the thickness of the polymeric layer.
11. The printhead of Claim 9 wherein the void spaces in the polymeric layer are between
the ink chambers and at least two opposing edges of the polymeric layer wherein the
void spaces have a depth which is at least about 80% of the thickness of the polymeric
layer.
12. The method of Claim 1, the printhead structure of Claim 7, or the cartridge of Claim
8 wherein the valleys/voids have a depth substantially equal to the thickness of the
polymeric layer.
13. The method of Claim 1, the printhead structure of Claim 7, or the cartridge of Claim
8 wherein the valleys/voids have a depth which is at least about 33% of the thickness
of the polymeric layer.
14. The method of Claim 1, the printhead structure of Claim 7, the cartridge of Claim
8, wherein the polymeric layer comprises a compound selected from the group consisting
of polydimethylglutarimide (PMGI)-based photoresists, polymethylmethacrylate (PMMA)-based
photoresists, PMGI-PMMA copolymer photoresists, photodecomposable polymeric compounds
derived from vinylketone, phenol-formaldehyde type photoresists and polyimide.
15. The printhead structure of Claim 7, the cartridge of Claim 8, or the printhead of
Claim 9 wherein the polymeric layer is comprised of at least two spin on polymeric
layers having a total thickness ranging from about 10 to about 30 microns.
16. The printhead of Claim 9 wherein the polymeric layer is comprised of a compound selected
from the group consisting of polydimethylglutarimide (PMGI)-based photoresists, polymethylmethacrylate
(PMMA)-based photoresists, PMGI-PMMA copolymer photoresists, photodecomposable polymeric
compounds derived from vinylketone, phenol-formaldehyde type photoresists and polyimide
and B-stageable adhesive having an overall thickness ranging from about 2 to about
50 microns.
17. A method for making an ink jet printhead comprising providing a semiconductor substrate
containing electrical tracing connected to energy imparting devices for ink on a surface
of the substrate;
applying a polymeric layer onto the surface of the semiconductor substrate, the polymeric
layer having a thickness;
treating the polymeric layer in one or more steps to provide ink chambers and ink
flow channels therein for flow of ink to the energy imparting devices and to produce
valleys adjacent the ink chambers, the valleys having aspect ratios such that valley
depths are less than the thickness of the polymeric layer; and
bonding a nozzle plate adjacent to the polymeric layer using heat to produce an ink
jet printhead,
wherein the valleys are of a size and located in an area of the polymeric layer
sufficient to minimise thermal stresses in the polymeric layer during the bonding
process.
18. The method of Claim 17 wherein the ink chambers and ink flow channels have aspect
ratios such that their depths equal the thickness of the polymeric layer.